The chloride ion /ˈklɔəraɪd/ is the anion (negatively charged ion) Cl⁻. It is formed when the element chlorine (a halogen) gains an electron or when a compound such as hydrogen chloride is dissolved in water or other polar solvents. Chloride salts such as sodium chloride are often very soluble in water. It is an essential electrolyte located in all body fluids responsible for maintaining acid/base balance, transmitting nerve impulses and regulating fluid in and out of cells. Less frequently, the word chloride may also form part of the "common" name of chemical compounds in which one or more chlorine atoms are covalently bonded. For example, methyl chloride, with the standard name chloromethane (see IUPAC books) is an organic compound with a covalent C−Cl bond in which the chlorine is not an anion.

A chloride ion is much larger than a chlorine atom, 167 and 99 pm, respectively. The ion is colorless and diamagnetic. In aqueous solution, it is highly soluble in most cases; however, some chloride salts, such as silver chloride, lead(II) chloride, and mercury(I) chloride are slightly soluble in water. In aqueous solution, chloride is bound by the protic end of the water molecules.

Sea water contains 1.94% chloride. Some chloride-containing minerals include the chlorides of sodium (halite or NaCl), potassium (sylvite or KCl), and magnesium (bischofite), hydrated MgCl₂. The concentration of chloride in the blood is called serum chloride, and this concentration is regulated by the kidneys. A chloride ion is a structural component of some proteins, e.g., it is present in the amylase enzyme.

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